The goal of this project is to define the molecular mechanisms involved in the replication of enveloped RNA viruses and in particular, to understand the factors which influence the regulation and expression of viral genetic information. The possible role of the viral RNA genome in directing virus assembly as well as certain aspects of viral RNA metabolism are being investigated in a system which uncouples murine leukemia virus RNA synthesis from terminal steps in virus maturation. The virus particles produced under these conditions resemble deletion mutants since they lack thA 70S RNA genome and contain predominantly 4S RNA. Another major interest concerns the mechanism by which murine leukemia virus particles encapsidate a selected population of host tRNAs. The possible role of one or more viral proteins in the tRNA selection process is being explored by analyzing the tRNA composition of several nonconditional viral mutants. Recent results indicate that the reverse transcriptase enzyme selects the tRNA primer used for viral RNA-directed DNA synthesis.